Three duplex modes of Frequency Division Duplex (FDD), Half-FDD (H-FDD) and Time Division Duplex (TDD) respectively are currently supported by a Long Term Evolution (LTE) system.
Particularly the FDD refers to uplink transmission and downlink transmission in different carrier frequency bands to allow concurrent signal reception and transmission for base station and User Equipment (UE) respectively; and the TDD refers to uplink transmission and downlink transmission in the same carrier frequency band to allow signal transmission and reception for base station and UE respectively in different periods of time.
One cell in LTE and earlier wireless communication systems is configured with only one carrier, and the bandwidth of a LTE system is up to 20 MHz, particularly as illustrated in FIG. 1.
In a Long Term Evolution-Advanced (LTE-A) system, the required peak rates of the system is up to 1 Gbps in the downlink and 500 Mbps in the uplink, as improved significantly over the LTE system. The required peak rates cannot be achieved with only one carrier of a maximum bandwidth of 20 MHz. It is thus necessary in the LTE-A system to extend the bandwidth available to the UE, and in view of this, the technology of Carrier Aggregation (CA) has been introduced where a plurality of consecutive or non-consecutive carriers from the same base station (eNB) are aggregated together to serve the UE concurrently with a desirable rate. These aggregated carriers are also referred to as Component Carriers (CCs). Each cell can be a component carrier, and cells (component carriers) from different eNBs cannot be aggregated. In order to ensure the UE of the LTE system to be able to operate in each of the aggregated carriers, the bandwidth of each of the carriers is not larger than 20 MHz, particularly as illustrated in FIG. 2. Four carriers can be aggregated by the base station of LTE-A as illustrated in FIG. 2, so that the base station can transmit data to the user equipment concurrently over the four carriers for an improved throughput of the system.
At present carrier aggregation across different systems cannot be supported in LTE, that is, an FDD carrier can only be aggregated with an FDD carrier, and a TDD carrier can only be aggregated with a TDD carrier.
In the LTE system, a radio frame is of 10 ms and a subframe is of 1 ms in both the FDD mode and the TDD mode. Seven TDD uplink-downlink configurations are defined for each radio frame of the TDD mode, particularly as depicted in Table 1, where D represents a DL subframe, U represents an UL subframe, and S represents a special subframe of the TDD system.
TABLE 1(TDD uplink-downlink subframe configurations)Uplink-downlinkSubframe indexconfiguration01234567890DSUUUDSUUU1DSUUDDSUUD2DSUDDDSUDD3DSUUUDDDDD4DSUUDDDDDD5DSUDDDDDDD6DSUUUDSUUD
In the FDD mode of the LTE system, the UE receives downlink data in subframe n−4 and feeds back signaling of whether the data in the downlink subframe needs to be retransmitted, that is, feeds back Acknowledgement/Negative Acknowledgement (ACK/NACK) information, also referred to as a Physical Downlink Shared Channel Hybrid Automatic Repeat ReQuest (PDSCH HARQ), in uplink subframe n. With carrier aggregation, ACK/NACK information corresponding to a plurality of downlink carriers in subframe n−4 will be fed back concurrently in uplink subframe n.
In the TDD mode of the LTE system, the UE may feed back, ACK/NACK information corresponding to a plurality of downlink subframes, in the same uplink subframe, that is, the UE detects transmission of a Physical Downlink Shared Channel (PDSCH), or a Physical Downlink Control Channel (PDCCH) indicating downlink semi-persistent scheduling to be released, in downlink subframe n−k and feeds back corresponding ACK/NACK information in uplink subframe n, where kεK, and values in the set K depend upon the TDD uplink-downlink configuration of the system and the particular subframe index, particularly as depicted in Table 2, where particularly for a special subframe of special subframe configuration 0 and 5 with a normal Cyclic Prefix (CP) or a special subframe of special subframe configuration 0 and 4 with an extended CP, and there is no ACK/NACK feedback for the special subframe, that is, the UE will not feed back ACK/NACK information for this special subframe.
TABLE 2(Related TDD downlink K values: K: {k0, k1, . . . kM−1})Uplink-DownlinkSubframe indexConfiguration01234567890——6—4——6—41——7, 64———7, 64—2——8, 7, 4, 6————8, 7, 4, 6——3——7, 6, 116, 55, 4—————4——12, 8, 7, 116, 5, 4, 7——————5——13, 12, 9, 8, 7, 5, 4, 11, 6———————6——775——77—
As depicted in Table 2, a plurality of radio frames are arranged in order, that is, if the last subframe in radio frame a is k, then the first subframe in radio frame a+1 is k+1, and Table 2 depicts values of K corresponding to the respective uplink subframes taking only one radio frame as an example, where n−k<0 indicates a downlink subframe in a preceding radio frame.
In the LTE system, Downlink Control Information (DCI) carried over a Physical Downlink Control Channel includes scheduling information, uplink power control information, etc., for downlink and uplink data transmission. The DCI intended for different uses and including different information contents can be in different formats, also referred to as DCI formats, a part of which may have a length dependent upon the system to which the DCI is applied, that is, the same DCI format may have different corresponding lengths in the FDD system and the TDD system because the different information fields or the different sizes of information fields specific to the different systems. For the sake of a convenient description, a DCI information field common to and with the same length in the FDD system and the TDD system will be referred to as a common information field, and a DCI information field specific to the different systems will be referred to as a dedicated information field. Generally the common information field includes the following information: a carrier indicator, allocation of resource blocks, a modulation and coding scheme, a new-data indicator, a transmit power indicator, pre-coding information, etc. The dedicated information fields includes:
(1) For DCI to schedule uplink data transmission:
1) There is no dedicated information field for FDD.
2) Dedicated information fields for TDD include:
An uplink (UL) index with a length of 2 bits, only in TDD uplink-downlink configuration 0, to indicate a scheduled uplink subframe.
A Downlink Assignment Index (DAI) with a length of 2 bits, in TDD uplink-downlink configurations 1 to 6, to indicate the number of scheduled downlink subframes among a plurality of downlink subframes for which ACK/NACK information is fed back in the same uplink subframe.
(2) For DCI to schedule downlink data transmission:
1) An FDD dedicated information field:
An HARQ process number with a length of 3 bits to indicate the serial number of a scheduled HARQ process.
2) TDD dedicated information fields:
An HARQ process number with a length of 4 bits.
A DAI with a length of 2 bits to indicate the number of scheduled downlink subframes among a plurality of downlink subframes for which ACK/NACK information is fed back in the same uplink subframe.
The DAI information field is introduced to address the problem of possible inconsistent understanding between the base station and the UE on the number of scheduled subframes when ACK/NACK information corresponding to a plurality of downlink subframes is fed back in a same uplink subframe in the TDD system, and also since at most eight HARQ processes are supported over a carrier in the FDD system whereas at most fifteen HARQ processes are supported in the TDD system, the HARQ process number information fields in the DCI for FDD and TDD are also different in length.
However carrier aggregation across different systems is not supported at present in LTE, so a design scheme has been absent so far for the respective information fields in the DCI for the TDD system and the FDD system with carrier aggregation. Aggregation of a TDD carrier and an FDD carrier may be supported in a later evolved LTE system. When a TDD carrier and an FDD carrier are aggregated, a Physical Downlink Shared Channel (PDSCH) HARQ feedback timing over the respective carriers is different from the original PDSCH HARQ feedback timing in the TDD system and the FDD system, so the use of the original DCI design scheme cannot be sufficient, for example, there will be a larger number of HARQ processes over an FDD carrier as a secondary carrier, and the larger number of HARQ processes cannot be supported with the existing HARQ process number information field in the FDD DCI.